The present invention relates to a method for manufacturing low cost ceramic parts and products with both improved machining efficiency and excellent reliability, and in particular, relates to a method that may be suitably used for the manufacture of complicatedly-shaped ceramic parts and products produced on a small volume, multiple production basis.
Ceramic materials such as silicon nitride, silicon carbide, partially stabilized zirconia and the like have excellent properties such as high resistance to heat, high resistance to abrasion, high hardness, and high resistance to corrosion, and have been advantageously used as parts of engines and other machine elements. In recent years, fields of the application of these ceramic materials have spread through successive improvements and rationalized designs.
Ceramic materials generally undergo contraction by ten and several percent when sintered, and must be machined or worked after sintered when they are used for such automobile engines and gas turbines that require dimensional accuracy.
Sintered ceramics are so hard that diamonds are used to work them by cutting. However, since ceramics are also such brittle materials that chipping and cracks are likely to form on their surface. Especially, cracks are found to form on the surface because of the heat shock due to the heat generated during such machine processing. Although various machining or working methods have been proposed, many of their practices are kept as proprietary in the form of "know-how" of private companies or organizations.
On the other hand, ceramic parts and products with complicated shapes such as turbo-charger rotors and turbine rotors are difficult to work and injection moulding is advantageously used and according to this method substantially finished shapes are obtained and machining or working before and after sintering is minimized.
Furthermore, Japanese Patent Laid-Open (Kokai) No. 59-96912 discloses a method for manufacturing ceramic products, wherein press-moulded materials are first calcined, machine-worked to the desired shape with a cutting tool having grinding particles adhered to it, then sintered, and afterwards machine-worked for the final shape.
Alternatively, rods of Al.sub.2 O.sub.3 and Si.sub.3 N.sub.4 are first produced by extrusion followed by calcining at various temperatures. Then, the resultant materials are worked dry by ultra-hard alloy ceramic bites, or worked wet by diamond bites. Reports on this method, in particular, about workability and abrasion of bites have been abundant.
The method of injection moulding has less working processes and may be suitably applied to moulding of turbocharger rotors and turbine rotors, but the cost of metal moulds is so expensive that this method may not be suitable for the manufacture of ceramic products of "small volume, multiple production".
In a dry method in which materials shaped by pressing or extrusion, or materials that have been removed of binders by heating are machine-worked dry, the bonding between particles of the moulded materials is so weak that they are susceptible to breakage. In addition, such particles are likely to be peeled off the worked surface, resulting in roughened surfaces and sometimes producing defective products with cracks. If grinding is used, small ground particles are loaded on the surface of the grinder and dressing must often be used, and working efficiency is substantially reduced. Furthermore, when it undergoes wet working, the moulded product tends to collapse while dry working is less efficient because of reduced cooling rates.
Further, Japanese Patent Laid-Open (Kokai) No. 59-96912 describes a method of calcining shaped materials at calcining temperatures ranging from 1000.degree. to 1500.degree. C. for 30 to 300 minutes to obtain hardness suitable to dry machining or working in which suitable temperatures for silicon nitride are in the range of 1470.degree. to 1490.degree. C. However, confirmation tests revealed that the hardness thus obtained was too high to attain satisfactory workability.
In addition, according to a method in which moulded Si.sub.3 N.sub.4 is calcined at a temperature of 1,000.degree. C. or above followed by cutting wet or dry, it has been reported that machineability is excellent when the moulded Si.sub.3 N.sub.4 is calcined at 1400.degree. C. while good surface roughness is obtained at calcining temperatures of 1300.degree. C. or lower. However, no suitable calcining conditions have been known for ceramic products such as turbo-charger rotors with complicated shapes which should be formed by machining followed by sintering.
The present invention is, therefore, to provide a method to solve the problem described above and is suitably used for the manufacture of low cost ceramic products with both improved machining efficiency and excellent reliability, and in particular, for manufacturing ceramic products that may be useful for a complicatedly-shaped ceramic product which are usually produced on a small volume, multiple production basis.